This is a bold claim, But I stand be­hind it: if you learn and fol­low these five ty­pog­ra­phy rules, you will be a bet­ter ty­pog­ra­ph­er than 95% of pro­fes­sion­al writ­ers and 70% of pro­fes­sion­al de­sign­ers. (The rest of this book will raise you to the 99th per­centile in both categories.)
All it takes is ten min­utes—five min­utes to read these rules once, then five min­utes to read them again.
Ready? Go.

The ty­po­graph­ic qual­i­ty of your doc­u­ment is de­ter­mined large­ly by how the body text looks. Why? Because there’s more body text than any­thing else. So start every project by mak­ing the body text look good, then wor­ry about the rest.
In turn, the ap­pear­ance of the body text is de­ter­mined pri­mar­i­ly by these four ty­po­graph­ic choices:

point size is the size of the let­ters. In print, the most com­fort­able range for body text is 10–12 point. On the web, the range is 15–25 pixels. Not every font ap­pears equal­ly large at a giv­en point size, so be pre­pared to ad­just as necessary.

line spacing is the ver­ti­cal dis­tance be­tween lines. It should be 120–145% of the point size. In word proces­sors, use the “Exact” line-spac­ing op­tion to achieve this. The de­fault sin­gle-line op­tion is too tight; the 1½-line op­tion is too loose. In CSS, use line-height.

line length is the hor­i­zon­tal width of the text block. Line length should be an av­er­age of 45–90 characters per line (use your word-count func­tion) or 2–3 low­er­case alphabets, like so:abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzabcd
In a print­ed doc­u­ment, this usu­al­ly means page margins larg­er than the tra­di­tion­al one inch. On a web page, it usu­al­ly means not al­low­ing the text to flow to the edges of the brows­er window.

And finally, font choice. The fastest, eas­i­est, and most vis­i­ble im­prove­ment you can make to your ty­pog­ra­phy is to ig­nore the fonts that came free with your com­put­er (known as sys­tem fonts) and buy a pro­fes­sion­al font (like those found in font recommendations). A pro­fes­sion­al font gives you the benefit of a pro­fes­sion­al de­sign­er’s skills with­out hav­ing to hire one.
If that’s im­pos­si­ble, you can still make good ty­pog­ra­phy with sys­tem fonts. But choose wise­ly. And nev­er choose times new roman or Arial, as those fonts are fa­vored only by the ap­a­thet­ic and slop­py. Not by ty­pog­ra­phers. Not by you.